This invention involves a neutral gray colored glass that has a luminous transmittance that makes it highly desirable for use as privacy glazing in vehicles, e.g. in the rear portions of vans. In particular, the glass of the present invention has a luminous transmittance range of up to 45 percent. The desired color and performance is attained by using iron, cobalt, selenium and/or nickel as colorants. In addition, the glass of the present invention generally exhibits lower infrared and total solar energy transmittance than typical green glasses used in automotive applications to reduce heat gain in the interior of the enclosure. The glass is also compatible with flat glass manufacturing methods.
Various heat-absorbing glass substrates are known in the art. The primary colorant in typical green tinted automotive glasses is iron, which is usually present in both the Fe.sub.2 O.sub.3 and FeO forms. As is conventional, the total amount of iron present in a glass is expressed herein as Fe.sub.2 O.sub.3, regardless of the form actually present. Typical green tinted automotive glass has about 0.5 percent by weight total iron with the ratio of FeO to total iron being about 0.25.
Some glasses, e.g. U.S. Reissue Pat. No. 25,312 to Duncan et al. and U.S. Pat. No. 4,339,541 to Dela Ruye, produce a gray color in the glass by including nickel oxide, cobalt oxide and selenium as colorants. However, inclusion of nickel-containing materials must be carefully controlled because the presence of nickel during the melting process sometimes leads to the formation of nickel sulfide stones in the glass. Additional potential problems faced when using nickel include haze formation on the glass surface due to reduction of the nickel in the tin bath and change in the glass color when it is heat treated.
To avoid this problem, nickel-free colored glasses containing iron oxide, cobalt oxide, and selenium were developed, as disclosed in U.S. Pat. No. 3,296,004 to Duncan et al., U.S. Pat. No. 3,723,142 to Kato et al. and British Patent Specification 1,331,492 to Bamford. In U.S. Pat. No. 4,104,076 to Pons, instead of nickel, Cr.sub.2 O.sub.3 or UO.sub.2 are used in combination with iron oxide, cobalt oxide, and selenium to produce gray glass. More recent nickel-free gray glasses are disclosed in U.S. Pat. No. 5,023,210 to Krumwiede et al. and U.S. Pat. No. 5,278,108 to Cheng et al. and EP 536 049 to Alvarez-Casariego, et al., which use iron oxide, cobalt oxide and selenium, as colorants. Krumwiede et al. and Alvarez-Casariego et al. further disclose that chromic oxide may be used as an additional colorant.
Many of the commercially available gray glasses are too dark to be used in the forward vision area of a vehicle. In addition, the glasses may also be too dark to allow safety lighting mounted in the interior of the vehicle from being seen from the exterior of the vehicle. It would be desirable to have a neutral gray glass that provides a dark gray color for use in privacy areas of a vehicle while allowing adequate safety lighting illumination and have a consistent color and further that is compatible with commercial flat glass manufacturing techniques.